It is common practice to rotatably support workpieces for the purposes of measuring the coordinates thereof or for the purposes of machining the workpiece. By way of example, workpieces are arranged on rotatable tables (so-called rotary tables) in the field of coordinate metrology. In this way, the workpiece can be brought into various work alignments, in which the coordinate measuring machine (abbreviated CMM) operates, i.e. measures coordinates of the workpiece. In particular, the coordinates of the workpiece can be measured continuously (e.g. in scanning fashion) while the rotary device rotates the workpiece about the axis of rotation thereof. Using the rotary device, the required measurement time/machining time can be shortened, the accuracy of the measurement/machining can be increased and/or a simpler coordinate measuring apparatus or a simpler machining tool can be used.
Corresponding statements apply to the machining of a workpiece by a machine tool. The workpiece can be brought into various work alignments in order to machine the workpiece. In particular, the workpiece can be rotated continuously while it is being machined.
In particular, the work alignment can be defined by a direction that extends perpendicular to the axis of rotation and through a point on the surface of the workpiece, at which the workpiece is sensed or at which the workpiece is machined. Therefore, the force acting on the workpiece during the tactile probing of the workpiece with a probe or during the machining of the workpiece can act, in particular, perpendicular to the axis of rotation in the direction of the work alignment.
In the field of coordinate metrology, for checking the shape of a workpiece, it is often advantageous to sense the workpiece with a probe which has an almost constant work alignment and work position relative to the rotary device while the rotary device rotates the workpiece. The work position and work alignment are not entirely constant, since the workpiece is generally not arranged exactly rotationally symmetrically with respect to the axis of rotation of the rotary device and/or is not, or not exactly, shaped rotationally symmetrically. By way of example, a probe of a coordinate measuring machine, which probes the surface of the workpiece in a tactile fashion, may be held by the coordinate measuring machine in a fixed position and with a fixed alignment, the probe being deflected to a different extent relative to a holder of the probe, depending on the workpiece shape to be measured. Owing to the almost constant work alignment and work position, errors of the coordinate measurement due to position-dependent and alignment-dependent errors of the coordinate measuring machine can be minimized. The errors of the rotary device in this case crucially determine the measurement result. The speed of the measurement of the workpiece can thereby also be increased in many cases.
Errors of the rotary device are caused by deviations of a real rotational movement from an ideal rotational movement. Eric Marsh describes in “Precision Spindle Metrology”, ISBN 978-1-932078-77-0, in particular Chapter 2, concepts for the description of movement errors of a precision spindle. The error separation method described by Marsh assumes a measurement design, in which three sensors simultaneously detect the movements of a rotating calibration body. The three sensors are held by a common holder. The measurement signals from the three sensors, recorded simultaneously, thereafter permit, by calculation, a separation of the shape errors of the calibration body (a test sphere) and the movement errors of the spindle.
The magnitude of the error of the rotary device in many cases depends on the introduced forces and moments, which are exerted by the mass of the workpiece arranged at the rotary device and/or by forces of a coordinate measuring apparatus or of a machining tool on the workpiece. Dynamic effects may also occur in the case of the rotational movement of the rotary device. At the location of the measurement of a workpiece, these errors are superposed on the errors of the workpiece to be measured, and so the errors of the workpiece cannot be measured exactly. When machining a workpiece, the errors of the rotary device lead to errors of the workpiece compared to a standard. Particularly large measurement errors or machining errors can be created in the case of, in particular, large workpieces due to the geometric amplification of the errors of the rotary device with increasing distance from the rotary device.
In order to reduce the errors of the rotary device, the rotary device may be designed in such a way that the error meets specifications. In particular, it is possible to use oil or air bearings for mounting the rotationally mobile parts of the rotary device, and in the case of motor-driven rotary devices, it is possible to use direct drives. The smaller the error of the rotary device is intended to be, the higher is the design outlay. The outlay for the production of such rotary devices is high and such rotary devices have large dimensions and they are mechanically complex and sensitive to external influences, such as dirtying, in many cases. In the case of arrangements for measuring coordinates or machining workpieces, which have roller bearings without need for a supply of air or oil, an oil or air bearing of the rotary device constitutes a significant additional outlay. In particular, the invention relates to coordinate measuring machines with such a roller bearing.
The reduction of errors of the rotary device by way of construction also leads to a restriction in the possibilities of use of the rotary device since the high mechanical accuracy of the rotational movement does not admit all desired fields of use. By way of example, rotary devices with an air bearing can only be loaded with restricted tilting moments and can therefore only rotate workpieces whose mass is not too great.
As an alternative or in addition, errors of the rotary device may be measured with a coordinate measuring machine, a calibration body or an arrangement of calibration bodies being arranged on the rotatable part of the rotary device (for example placed on the rotary table) and measured. However, measuring the errors of the rotary device in respect of all six possible degrees of freedom of movement is time-consuming. If a high accuracy is required, the calibration needs to be repeated, for example when the rotary device is subjected to temperature variations. Corresponding considerations apply for a rotary device which is configured in order to hold workpieces rotatably in the machining range of a machine tool. The outlay for calibration is then usually even greater compared with coordinate metrology, since in the field of coordinate metrology the coordinate measuring machine which subsequently carries out the measurement of workpieces can mostly also be used for the calibration.
It is an object of the present invention to specify a method for reducing errors of a rotary device when determining coordinates of a workpiece or when machining a workpiece, which method requires little measurement and structural outlay in order to keep the error of the rotary device low. In particular, the ways of implementing a method for measuring a workpiece or for machining a workpiece already described above should be possible with little outlay. Furthermore, it is an object of the present invention to specify an arrangement for carrying out the method.